1. Field of the Invention
The present invention relates generally to a lens barrel of a camera or an interchangeable lens detachably attached to the camera and, more particularly, to a lens fixing structure in the lens barrel, which is used for fixing and holding a plurality of lens elements to a lens frame.
2. Related Background Art
A lens barrel incorporates a single or a plurality of lens units as a photographing optical system. It has hitherto been practiced that a lens assembly body is assembled by the plurality of lens elements constituting the above lens unit and a lens frame incorporating and holding these lens elements.
As a method of incorporating, fixing and thus holding the plurality of lens elements of the lens unit at a predetermined interval into the lens frame in the lens barrel described above, there is known a first method of, as illustrated in FIG. 5, incorporating the lens elements into the lens frame from one direction, and interposing a spacing ring pre-worked to have a predetermined thickness enough to secure a predetermined spacing between the lens elements.
More specifically, referring to FIG. 5, a lens assembly body 1 is so disposed as to be movable or fixed in an unillustrated lens barrel and constitutes lens units of a photographing optical system. The lens assembly body 1 includes a lens frame 2, and two lenses 3, 4 incorporated into this lens frame 2.
Herein, FIG. 5 illustrates a case where the two lenses are sequentially incorporated into the lens frame 2 from one side. A space ring 5 is interposed between these lenses 3, 4, and a stop ring 6 is screwed through a screw portion 6a to an incorporating-side aperture edge of the lens frame 2, thus fixing the whole. Note that an inside diameter flange portion designated by 2a in FIG. 5 is formed along a non-incorporating-side aperture edge of the lens frame 2.
There is known also a second method of securing the spacing between the lenses 3, 4 owing to a thickness of a flange wall 7 by providing, as shown in FIG. 6, this flange wall 7 having a predetermined thickness at the center of the lens frame 2 in the optical-axis direction, and incorporating and fixing the lenses 3, 4 on both sides of this wall 7.
Herein, a stop ring indicated by the numeral 8 in FIG. 6 has a screw portion 8a through which the stop ring 8 is screwed to the incorporating-side aperture edge of the lens 3 to fix the lens 3 incorporated into the lens frame 2.
In the lens assembly body 1 used for the prior art lens barrel described above, however, when fixedly holding the lenses 3, 4 in the lens frame 2 by the first method, a gap must be needed between the space ring 5 and an inner peripheral wall of the lens frame 2. Therefore, if the space ring 5 gets eccentric, the problem arises in which a contact point to the lens 4 is inclined enough to let the lens 4 down. Further, if an accuracy of a thick dimension of the space ring 5 falls within an allowable error range on the order of, e.g., several tens .mu.m, the working of the space ring 5 is easy. If the accuracy as precise as several .mu.m is required, however, there might arise a problem in which the working method is difficult, and the costs increase.
Moreover, if the lens frame 2 is molded into a resinous molding by the second method described above, it is highly difficult to secure an accuracy of the thick dimension of the flange wall 7 to determine the spacing between the lenses 3, 4.
That is, a molding die for molding the lens frame 2 is, as shown in FIG. 7, constructed of a pair of dies 9A, 9B, and a cavity (indicated by 2A in FIG. 7) for forming the lens frame 2 is formed between these dies 9A, 9B. There must be, however, a difference between the die 9A for forming one chamber with the flange wall 7, interposed therebetween, for keeping the spacing between the lenses 3, 4 with a predetermined thick dimension, and the die 9B for forming the other chamber. Consequently, an error in thickness of the wall 7 is twice as large as an error in working of the dies 9A, 9B. Further, there must be a differences between dies for forming a contact points of the lens 3, an inside diameter portion 7d, a contact point 7a of the lens 4, and an inside diameter portion 7c, and hence a deviation is produced in the direction perpendicular to the optical axis when matching the dies enough to cause eccentricity, resulting in a discrepancy in the optical axes of the lenses 3, 4.
Accordingly, according to these prior art methods, it is extremely difficult to keep the error in the spacing accuracy between the lenses 3, 4 within an error range on the order of several .mu.m even when using the space ring 5 or the flange wall 7. It has been desired to take some measures capable of obviating the problem in terms of the accuracy.